Aromatic polyimide films show high heat resistance, good chemical resistance, high electrical insulating property and high mechanical strength, and therefore are widely used in various technical fields. For instance, an aromatic polyimide film is favorably employed in the form of a metal-coated continuous film for manufacturing a flexible printed circuit board (FPC), a carrier tape for tape-automated-bonding (TAB), and a tape of lead-on-chip (LOC) structure.
Previously, the metal-coated aromatic polyimide film has been produced generally by bonding an aromatic polyimide film to a metal foil using a conventional adhesive such as epoxy resin. However, due to low heat-resistance of the conventional adhesive, the produced metal-coated aromatic polyimide film cannot show satisfactory high heat-resistance. It is difficult particularly for an aromatic polyimide film containing biphenyltetracarboxylic acid units, for instance, UPILEX-S (available from Ube Industries, Ltd.) which comprises biphenyltetracarboxylic acid units and phenylenediamine units, to fix directly a metal foil onto the film due to poor affinity of the surface of the polyimide film. Moreover, the metal foil is not appropriately employable for producing a metal-coated aromatic polyimide film having a thin metal coat which is strongly demanded at present. The thin metal coat is favorably employable for producing a circuit of a fine etched pattern.
In consideration of the above-mentioned problem and present demand, it has been proposed to manufacture a metal-coated aromatic polyimide film by forming a thin copper metal film on an aromatic polyimide film by means of electro-plating. However, it is difficult to produce an aromatic polyimide film having a well fixed metal film due to the poor affinity of the polyimide film.
It has been known that a surface of an aromatic polyimide film can be improved on its affinity to metal plating or deposition, by processing the film surface with an alkaline solution. However, the wet processing is not industrially preferred because it is required to sufficiently wash the alkaline-processed surface with pure water. Surface plasma treatment and corona discharge treatment are also known to improve the affinity of the polyimide film. However, such treatments are not satisfactory to produce a metal-coated polyimide film having high separation resistance between the metal film and the polyimide film.
Japanese Patent Provisional Publications No. 6-124978 and No. 6-210794 describe to coat a layer of PMDA polyimide which is produced from a combination of pyromellitic dianhydride and 4,4′-diaminodiphenyl ether over such an aromatic polyimide film as UPILEX-S and then depositing or plating a metal such as copper on the PMDA polyimide layer. Thus produced metal-coated aromatic polyimide film, however, has a problem in that the PMDA polyimide layer is not fixed onto the UPILEX-S with satisfactory bonding strength.
Japanese Patent Provisional Publication No. 1-321687 describes a process for preparing a flexible printable circuit board to process a PMDA polyimide film (such as a commercially available KAPTON) with glow-discharge plasma to increase the surface tension of the film to 54 dyne/cm or higher, depositing a metal layer on the processed surface of the film under vacuum, and forming a thick copper film on the metal-deposited layer by plating. According to the working example, however, the initial peeling strength of the produced metal-coated polyimide film is in the range of 0.5 to 0.8 Kgf/cm which is not satisfactorily high. The present inventors have found that a metal-coated polyimide film which is produced from an aromatic polyimide film containing 3,3′,4,4′-biphenyltetracarboxylic acid units (e.g., UPILEX-S) in place of the PMDA polyimide film by the above-mentioned process shows an initial peeling strength of less than 0.5 Kgf/cm.